Do you mean how large of a value? If you do then check the limits.h library

hmm.. nope ?
im trying to check how many digit of user can be enter..
like example of my code above..
is only allow 2-digits.. if you enter 123 you'll get into the error message(while loop)
else if you enter 12 or 1 you will not be inside the loop..

printf("Which Years Are You Join?(2-digits) ");
scanf("%d", &Student[i].regNo.year);
while(Student[i].regNo.year < 100)
{
printf("You Have Enter Out of Range\n");
printf("Please Re-enter Again\n");
printf("Which Years Are You Join?(2-digits) ");
scanf("%d", &Student[i].regNo.year);
}

Since the user is allowed to enter any 1- or 2-digit value, the user allowed to enter any value between 0 and 99 (including those values). 100 won't be allowed.

EDIT: I forgot to mention that the value that is enter might be negative (if the 'year' variable is signed). To prevent a negative value we can use one additional check: Student.regNo.year > 0 in the while loop.